The present invention relates to characterizing free-space electromagnetic radiation using electro-optic or magneto-optic crystal sampling, and more particularly, to electro-optic or magneto-optic measurement of a spatial-temporal distribution of free-space pulsed radiation using a chirped optical pulse.
Electro-optic sampling is a powerful technique for the characterization of a repetitive electrical waveform, such as an electrical signal in an integrated circuit (see Kolner et al., IEEE J. Quantum Electron., Q.E.-22, p. 69 (1986), and Valdmanis et al., IEEE J. Quantum Electron., Q.E.-22, p. 79 (1986)), or a terahertz beam in a free-space environment (see United States applications by Zhang et al., entitled xe2x80x9cElectro-Optical Sensing Apparatus and Method for Characterizing Free-Space Electro-Magnetic Radiation,xe2x80x9d Ser. No. 08/739,099, now U.S. Pat. No. 5,952,818, and xe2x80x9cElectro-Optical and Magneto-Optical Sensing Apparatus and Method for Characterizing Free-Space Electro-Magnetic Radiation,xe2x80x9d Ser. No. 08/902,561, now U.S. Pat. No. 6,111,416, both of which are hereby incorporated herein by reference).
Conventional time domain electro-optic sampling is based on the repetitive property of the signal to be tested. A sequential plot of the signal versus time delay reassembles the temporal form. Unfortunately, if the signal to be measured is from a single-event experiment, such as an explosion or transitory breakdown, this technique is clearly not suitable.
Time-domain optical measurements, such as the terahertz time-domain spectroscopy in pump/probe geometry of the above-incorporated United States Patent Applications, use a mechanical translation stage to vary the optical path between the pump and the probe pulses. The intensity or polarization of the optical probe beam, which carries information generated by the pump beam, is repetitively recorded for each sequential time delay. In general, this data acquisition for the temporal scanning measurement is a serial acquisition; i.e., the signal is recorded during the probe/pulse sampling through a very small part of the terahertz waveform (roughly the pulse duration of the optical probe beam). Therefore, the data acquisition rate in this single channel detection approach is limited to less than 100 Hz for a temporal scan on the order of tens of picoseconds. Clearly, this relatively low acquisition rate cannot meet the requirement for real-time measurements, such as time-domain terahertz spectroscopy, of fast-moving objects or flame analysis.
Thus, there exists a need in the art for an enhanced technique for measurement of a terahertz spatial-temporal distribution, and particularly for one which is suitable for real-time imaging applications using free-space radiation.
Briefly summarized, in one aspect a sensor for characterizing free-space radiation is presented. The sensor includes one of an electro-optic crystal or a magneto-optic crystal positionable so that the free-space radiation passes therethrough. Means are provided for generating a chirped optical probe signal and for co-propagating the chirped optical probe signal through the crystal with the free-space radiation such that a temporal waveform of the free-space radiation is encoded onto a frequency spectrum of the chirped optical probe signal. The sensor also includes means for decoding a characteristic of the free-space radiation using the chirped optical probe beam with the temporal waveform of the free-space radiation encoded on its frequency spectrum.
In another aspect, an imaging system for imaging an object is provided. The imaging system includes means for generating a free-space electromagnetic radiation pulse positionable to pass through the object to be imaged, and one of an electro-optic crystal or a magneto-optic crystal positioned so that the electromagnetic radiation pulse passes through the crystal after passing through the object. The system further includes means for generating a chirped optical probe signal to impinge the crystal simultaneous with the electromagnetic radiation pulse passing therethrough so that a temporal waveform of the radiation is encoded onto a wavelength spectrum of the chirped optical probe signal. The chirped optical probe signal modulated by the free-space radiation is then passed to decoding means for decoding a characteristic of the free-space electromagnetic radiation using the chirped optical probe signal with the temporal waveform of the radiation encoded thereon. The system further includes means for determining a characteristic of the object using the characterization of the free-space electromagnetic radiation pulse after passing through the object.
In a further aspect, a method is provided for characterizing free-space radiation. The method includes: providing one of an electro-optic crystal or a magneto-optic crystal positionable so that the free-space radiation passes therethrough; generating a chirped optical probe signal and co-propagating the chirped optical probe signal through the crystal with the free-space radiation so that a temporal waveform of the free-space radiation is encoded onto a wavelength spectrum of the chirped optical probe signal; and decoding a characteristic of the free-space radiation using the chirped optical probe signal with the temporal waveform of the free-space radiation encoded on its wavelength spectrum.
To restate, presented herein is a measurement technique employing a chirped optical probe beam which allows characterization of a free-space electromagnetic pulse. When the chirped optical probe beam and radiation pulse co-propagate in an electro-optic or magneto-optic crystal, different portions of the radiation pulse, through Pockels effect, modulate different wavelength components of the chirped pulse. The resultant modulated spectral distribution can then be decoded by taking the difference between the modulated spectrum and the spectral distribution of the chirped optical probe without the radiation modulation. The measurement technique of this invention provides single-shot measurement ability and ultrafast measuring speed. With these advantages, the technique can be employed with a number of possible applications, including monitoring for transient emitter breakdown, measuring unsynchronized a microwave, spatial-temporal imaging of non-terahertz signals, monitoring various unsynchronized fast phenomenon, such as chemical reactions and explosions, and studying non-linear effects. Various additional advantages will also be apparent to those skilled in the art based upon the embodiments of the invention presented hereinbelow.